1. Field of the Invention
The present invention relates to a method for recovering optically active tryptophan from a fermentation broth or solution containing the same.
2. Description of Prior Art
Optically active tryptophan has been produced by use of tryptophan-producing bacterial belonging to the genus Bacillus, the genus Brevibacterium, the genus Corynebacterium, the genus Arthrobacter, and other genera, using glucose or the like as a main starting material. However, crude crystals of optically active tryptophan obtained by conventional methods contain impurities such as proteins, coloring matter, etc.
These impurities are generally very hydrophobic, so that even though recrystallization is performed in a conventional manner, the impurities are attracted to the surface of hydrophobic tryptophan crystals, or are occluded onto these crystals. Even if the amount of the impurities is small, purification is difficult. To remove these impurities, the prior art has suggested several methods including:
(1) contacting a solution of crude tryptophan crystals with a nonionic exchange resin to adsorb the impurities onto the resin, and then filtering through an ultrafiltration membrane (Japanese Patent Unexamined Published Application No. 58-895); or
(2) heating a tryptophan enzyme reaction solution at 95 to 100.degree. C. in the presence of activated carbon under acidic pH conditions, then performing a solid-liquid separation, contacting the filtrate containing tryptophan with a non-polar porous resin to concentrate and then adding an aliphatic lower alcohol (Japanese Patent Unexamined Published Application No. 61-126070); or
(3) adjusting the pH of a tryptophan-containing solution to an alkaline region upon crystallization and performing neutralization crystallization in the presence of a lower alcohol or ketone (Japanese Patent Unexamined Published Application Nos. 59-39857, 60-30694 and 61-12607); etc.
However, none of these methods for purification have proven to be completely satisfactory because:
(1) Since tryptophan is molecularly adsorbed onto resins due to its specific affinity, attributable to its molecular structure, tryptophan has a tendency to be adsorbed and desorbed together with such impurities as coloring matters, etc. It is quite complicated to separate these impurities, and hence, the yield is adversely affected.
(2) Tryptophan has an indole ring which is structurally unstable. If one tries to remove the impurities with any process employing high temperatures, degradation can occur.
(3) Crystallization of tryptophan using an organic solvent is not efficient, since impurities are not easily removed. Further, it is complicated and dangerous to handle and recover organic solvents.